PI ^ AU9817130 IONOLUMINESCENCE (IL) OF SYNTHETIC DIAMONDS.

A. A. Bettiol, K. W. Nugent, D. N. Jamieson and S. Prawer

School of Physics, Microanalytical Research Centre, University of Melbourne, Parkville, 3052, AUSTRALIA. Introduction

The optical properties of natural and synthetic diamonds have been extensively characterized in the past by absorption and luminescence. The use of such techniques as cathodoluminescence, photoluminescence, photoluminescence excitation and electron spin and paramagnetic resonance has resulted in the identification of many impurity and defect related optical centres in diamond [1-2]. Of the impurities found in diamond, nitrogen is by far the most abundant and hence responsible for most of the optical properties [3]. The development of diamond synthesis methods has resulted in the discovery of a number of a new optically active impurities and defects which are introduced during the growth process. These include Si, O, Ni and B [1-2]. In this study we identify a number of defect and impurity related centres in two commercially produced synthetic diamond samples by using the novel technique of ionoluminescence (IL) [4].

The first sample characterized is a Norton polycrystalline diamond detector. Signal produced in any charged particle detector is degraded if recombination of the electrons and holes occurs before the charge can be swept out by the electric field in the detector. In diamonds where the radiative recombination cross-section can be quite high, signal degradation can occur depending on the optical centres present and their lifetimes. Recombination centres with lifetimes much longer than the sweep out time will potentially saturate hence only cause a degradation of signal. Recombination centres with short lifetimes can cause severe signal loss since electron hole pairs will all recombine before they can be swept out by the electric field.

A synthetic diamond sample produced by Sumitomo is also analyzed to look at impurity related centres.

Experiment

A beam of 3 MeV protons was used to analyze the two diamond samples which were cooled down to 30K. To reduce beam damage effects, the beam was focused down to about 5 microns and then scanned over an area of 200 microns. Light excited by the ion beam was collected by a microscope mounted at 45 degrees to the beam direction. This light was coupled to an optical fibre and then diverted to a number of detectors. The first of these is an Ocean Optics SD1000 CCD array spectrometer with a 600 I/mm, 500nm blaze grating. The 100 \im core HOH-UV optical fibre used for light collection provides a 5 nm resolution over a wavelength range of 360 - 750 nm. High resolution data is acquired through a 200 ixra core LOH fused silica fibre which is then coupled to a DILOR xy triple monochromator with an EG& G PARC OMA4 liquid nitrogen cooled CCD array detector. A resolution of 0.1 nm can be readily achieved over the visible region of the spectrum. For imaging the light was diverted to a monochromator coupled to a Hamamatsu R943-02 photomultiplier tube. Samples were cooled down to a temperature of 30K by a liquid helium cryostat. Temperature monitoring is achieved with the aid of two Lakeshore DT-470 silicon diode sensors mounted at the cold head and on the target holder. Results and Discussion

Two peak are observed in the low resolution spectrum from the Norton polycrystalline diamond detector sample at low temperature (figure la). The dominant feature is the broad A-band luminescence in the blue part of the spectrum. This emission has been well studied in the past and is attributed to donor-acceptor pair recombination [5]. A smaller peak observed at about 500 nm is resolved into two components by the high resolution spectrometer (Figure lb). The most intense of the two peaks is a sharp zero phonon line at 2.462 eV which can be tentatively attributed to the 3H centre in diamond [6]. The second peak is somewhat broader than the 3H centre and has a measured energy of 2.477 eV. Another weak zero phonon line appears at 2.563 eV (not shown), superimposed on the A-band emission. The 2.477 eV and the 2.563 eV are difficult to assign as their positions don't exactly coincide with previously observed CVD diamond emission lines.

250 •

• 8 (orb . units ) I/ \ |ioo I \ 50 J

2.2 2.4 2.6 2.8 3.0 3.2 3.4 2.62 2.43 2.44 2.45 2.46 2.47 2.48 2.49 So Energy <«V>

(a) (Fig 1:IL spectra - Norton diamond) (b)

Monochromatic IL images taken at wavelengths corresponding to the A-band (420 nm) and the three observed emission lines (484 nm, 500 nm, 503.5 nm) do not show any difference in contrast. This indicates that the luminescent regions in the polycrystalline diamond film have identical properties. Figure 2 shows monochromatic maps at 420nm and 500nm for a scan size of approxi- mately 800x800 micron (figure 2a) and 200x200 micron (figure 2b). In these maps, white indicates high luminescence intensity.

420nm 500nm 420nm 500nm

(a) (Fig 2:IL maps - Norton diamond) (b)

Point spectra were taken from two regions on the Sumitomo synthetic diamond. A low resolution spectrum of the central region shows a broad band in the green part of the spectrum (figure 3a). In high resolution we find a zero phonon line at 2.464 eV which is attributed to the H3 centre (figure 3b) [7]. The origin of the two other lines which appear in the spectrum is not clear. The 2.495 eV line is possibly due to the S3 diamond centre. The presence of a small satellite peak at 2.490 eV is however not consistent with the S3 centre. 12 2.4 2.6 2.46 2.48 En«rgy («V> Enargg C«V)

(a) (Fig 3:IL spectra - Sumitomo diamond) (b)

Z53 2.S4 En.rgg (tV>

(a) (Fig 4:IL spectra - Sumitomo diamond) (b)

High resolution analysis performed on the {111} growth sector of the Sumitomo diamond revealed a wealth of structure which can all be attributed to the presence of Ni in the sample. The first band observed (figure 4a) is made up of five sharp components with zero phonon lines at 2.556 eV, 2.557 eV, 2.561 eV, 2.565 eV and 2.570 eV. Associated with these lines are two broad peaks at 2.525 eV and 2.537 eV. A zero phonon line doublet, also due to optically active Ni, was observed at 1.401 and 1.404 eV (figure 4b) [8].

Conclusion

A number of optical centres have been identified in two commercially produced synthetic diamond samples. Future studies on Norton diamond detector samples will attempt to correlate these opti- cal centres with charge collection efficiency which can be measured insitu by the ion beam induced charge (IBIC) technique. Results obtained on the Sumitomo diamond sample are consistent with previous studies which have shown that Ni impurities segregate in {111} growth sectors.

References

[1] J. Walker, Rep. Prog. Phys, 42 (1979) 1605-1659 [2] J.E. Field (ed), The properties of Natural and Synthetic Diamond, Academic Press, London, (1992) pp35-79 [3] G. Davies and I. Summersgill, Diamond Research 1973 (London: Industrial Diamond Information Bureau) pp6-15 [4] A.A. Bettiol, D.N. Jamieson, S. Prawer and M.G. Allen, Nucl. Instr. Meth B 85 (1994) 775-779 [5] P.J. Dean, Phys. Rev. 139 (1965) 588-602 [6] A.T. Collins, M. Kamo and Y. Sato, J. Mater. Res, 5 (1990) 2507-2514 [7] A.T. Collins, H. Kanda and R.C. Burns, Phil. Mag. B61 (1990) 797-810 [8] V.A. Nadolinny, A.P. Yelisseyev, Diamond and Related Materials. 3 (1993) 17-21

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